Include en-US description of custom roles in static API

[dtolnay]

In #1154, the static-api data (v1/teams.json) included only ids of roles, such as "roles": ["spec-editor"]. Separately the public-facing descriptions for the roles would be included in a ftl translation file produced by dump-website.

governance-role-spec-editor = Editor

This worked!

but has the downside that if some new role is introduced into the team repo before the translation for it lands in rust-lang/www.rust-lang.org in locales/en-US/teams.ftl, then the website would temporarily display the following ugly placeholder, until a commit such as rust-lang/www.rust-lang.org#1821 syncs the ftl file.

There are 2 potential fixes:

1. Hide role from the website if localization is not available. This means roles would be added in rust-lang/team without immediately showing up on the website. They would show up only after the next ftl sync, which happens only about once a year.

2. Include an en-US description in static-api. As soon as a role appears in rust-lang/team, it will appear on the website with a fallback to the English description from teams.json.

Approach 2 is what the website already does for team names and descriptions. See the logic here: https://github.com/rust-lang/www.rust-lang.org/blob/b35215187ed75d4d2a9a056f1847309aa2d404d2/src/i18n.rs#L151-L169

Pseudocode:

if lang == "en-US" {
    write(team.website_data.description)
} else if let Some(localized) = fluent::get("governance-team-" + team.name + "-description") {
    write(localized)
} else {
    write(team.website_data.description)
}

This PR makes it possible to do approach 2 for roles too.

{
  "name": "spec",
  "members": [
    ...
    {
      "name": "Joel Marcey",
      "github": "JoelMarcey",
      "roles": [
        "spec-editor"
      ]
    }
  ],
  "website_data": {
    "name": "Specification team",
    "description": "Creating and maintaining the specification for the Rust language",
    "page": "spec",
    ...
  },
  "roles": [
    {
      "id": "spec-editor",
      "description": "Editor"
    }
  ]
}

[rylev]

This seems reasonable to me!